Zinc phosphate conversion-coatings have been applied to car and truck bodies for well over 50 years to provide corrosion protection and an adhesion base for paint. These coatings, in conjunction with the electrocoat (E-coat) primer, provide most of the corrosion protection for cold rolled steel and virtually all of the paint adhesion properties to both cold rolled steel and galvanized steel. The zinc phosphate conversion-coatings are deposited by electrochemical reaction of the metal substrate with an acidic, aqueous solution of metal phosphates carefully adjusted to a pH generally between 2.7 and 3.2. A typical, widely used, commercial zinc phosphate solution for automotive uses having this pH range contains hydrogen, zinc, and nickel cations; monohydrogen and dihydrogen phosphates; nitrate and fluoride anions; and soluble phosphate complexes of zinc and nickel. An accelerator, such as nitrite, which facilitates the solution of the iron surface and removal of H.sub.2, is continuously added to the solution in order to accelerate the electrochemical reaction during the phosphating operation.
Optimum phosphate coatings are only obtained if the components of the phosphate bath are maintained within the specific narrow limits designated for each constituent. As metallic parts are immersed or sprayed in large scale phosphating operations, the coating deposition process removes nickel, zinc, and phosphate from a bath and reduces bath acidity. Constituent monitoring and replenishment must keep pace with the depletion rate. Current industrial practice is to monitor only three bath parameters: free acid (FA), total acid (TA), and the nitrite accelerator, and to do so by manual titration. The bath is then replenished based on these parameters. The concentration of bath constituents such as nickel, zinc, and fluoride, all of which effect the coating quality, are maintained by addition of a mixed ion concentrate based on the free and total acid levels being monitored. The mixed ion concentrate is prepared assuming a depletion rate that is a unique function of the change in free and total acid. This assumption is generally not valid since all the components are not depleted at a constant rate because coating composition varies with the line speed, temperature, metal surface reactivity, metal mix, and other factors. From studies of such baths, we have found that the total phosphate result, currently derived from total acid count, is not accurate due to the presence of phosphate-metal complexes. Additionally, we have found some wide fluctuations in the zinc and nickel levels even though the current control methods show the baths to be operating within the process specification for free acid, total acid and nitrite.
It has been found that to insure that precise bath compositions are maintained it is necessary to quantitatively analyze the following zinc phosphate bath parameters: PH, total phosphate (mathematically related to total phosphorus and total acid), nitrite, and zinc concentrations. The concentrations of other component ions such as nickel and fluoride, when used, preferably also need to be quantitatively analyzed. Based upon such analysis, a precise bath composition can be maintained, a rigorous requirement for the application of consistently high quality coatings from phosphate baths in use at the present time.